How to create a bootable installer for macOS

You can use an external drive or secondary partition as a startup disk from which to install the Mac operating system.

A bootable installer can be useful when you want to install macOS on multiple computers without downloading the installer each time, or without connecting to the Internet.

These advanced steps are primarily for system administrators and others who are familiar with the command line. You don't need to create a bootable installer to install macOS.

Download macOS from the App Store

Download macOS from the Mac App Store. It downloads to your Applications folder as a single ”Install” file, such as Install macOS High Sierra.If you're creating a bootable installer for macOS High Sierra, download from a Mac using High Sierra, Sierra 10.12.5 or later, or El Capitan 10.11.6. Enterprise administrators: Please download from Apple, not a locally hosted software-update server.

When the installer opens, quit it without continuing installation.

Then use the 'createinstallmedia' command in Terminal

After downloading the installer, mount the USB flash drive or other volume that you're using for the bootable installer. Make sure that it has at least 12GB of available storage.

Open Terminal, which is in the Utilities folder of your Applications folder.

Type one of the following commands in Terminal. These all assume that the installer is in your Applications folder, and the name of the volume that you're using for the bootable installer is MyVolume. If your volume is named differently, replace MyVolume with the name of your volume.